The present invention relates to particles for an electronic-paper display device and a method for preparing the same.
As information technology rapidly develops and information visualization is required, each kind of display devices for replacing the conventional cathode ray tube (CRT) display device is being developed. The display devices are being developed to be thinner and lighter and to have a larger screen.
As the display devices, there are a liquid crystal display device, a plasma display device, an organic electroluminescent display device, a field emission display device, an electronic paper display device, etc.
Among the display devices, the electronic paper display device refers to a liquid crystal display device for providing a user a thin and flexible feeling like paper, in which charged particles are moved and rotated by applying a voltage to upper and lower substrates. The electronic paper display device is being spotlighted since it can provide a clear image with less driving power for a long time when compared with other display device.
The electronic paper display device uses an electrophoresis principle by which a conductive material has kinetic energy by receiving an electromagnetic field. More concretely, charged particles float in a certain space between two substrates that are flexible and thin, and each polarity of the charged particles is changed thereby to arrange the charged particles in a certain direction.
Fluid having the same density as the charged particles is filled in the space where the charged particles float, that is, inside a pixel. Accordingly, the charged particles electrophoresed in a certain direction by the polarity change thereof are scarcely influenced by gravity. Therefore, the charged particles electrophoresed in a certain direction are maintained as it is even if power is cut off. That is, an image displayed on a display device is maintained by a memory effect, thereby obtaining a similar effect to an effect that ink is printed on a paper. The displayed image is maintained for a long time unless it is reset by a controller, so that it has a low consumption power and can be utilized as a portable display device.
The conventional electronic paper display device is shown in FIG. 1.
In the conventional electronic paper display device 10, a partition 14 is positioned between an upper substrate 12 where a transparent electrode 13 is formed and a lower substrate 11 thereby to form a plurality of cells (pixels). Negatively-charged particles 15A and positively-charged particles 15B are positioned in the pixels formed by the partition 14, and float by the fluid 16 having the same density as the particles 15A and 15B. The lower substrate 11 and the upper substrate 12 have a thin thickness of several tens to several hundreds of μm, and the particles 15A and 15B electrophoresed between the lower substrate 11 and the upper substrate 12 have a very small size. The transparent electrode 13 has a thickness much thinner than that of the upper and lower substrates 11 and 12.
An operation of the conventional electronic-paper display device 10 will be explained. When different voltages (−, +) are applied to upper and lower portions of the pixels where the negatively-charged particles 15A and the positively-charged particles 15B float, that is, the transparent electrode of the upper substrate and the transparent electrode of the lower substrate, the negatively-charged particles 15A are electrophoresed in a positive electrode direction and the positively-charged particles 15B are electrophoresed in a negative electrode direction. Under an assumption that the negatively-charged particles 15A and the positively-charged particles 15B are colored with different colors (for instance, black and white), a certain image is displayed by the color difference between the charged particles 15A and 15B electrophoresed in different directions by voltages (−, +) applied to the transparent electrode 13.
In the conventional electronic paper display device, a mobility characteristic and a uniform electrification capacity of the charged particles by an electrophoresis have to be obtained, and a charged state of the particles has to be maintained.
As shown in FIG. 2, the particles 150 used in the conventional electronic paper display device are composed of a high molecule resin 151, a colorant 152, and a charge controlling material 153. Also, an external additional material 154 is coated on the surface of the charged particles 150 in order to obtain an electrophoresis characteristic, etc.
In the particles 15A and 15B, a position of the chare controlling material 153 included in the high molecule resin 151 is important.
When the charged particles 15A and 15B are electrophoresed, a moving speed and a moving amount of the charged particles 15A and 15B are determined by an electrification capacity thereof, and the electrification capacity of the charged particles 15A and 15B is determined by an amount of the charge controlling material 153 positioned near the surface of the particles 15A and 15B. That is, a response speed, a resolution, etc. of the electronic paper display device are more influenced by the charge controlling material 153 positioned near the surface of the particles rather than the charge controlling material 153 positioned inside the particles 15A and 15B.
Since the particles 15A and 15B are formed as fine grain, it is almost impossible to evenly distribute the charge controlling material 153. That is, according to the conventional method, it is impossible to implement the most preferable position of the charge controlling material 153, that is, the position on the surface of the particles 15A and 15B rather than inside the particles 15A and 15B. Since the charge controlling material 153 is unevenly distributed in the particles 15A and 15B, or masses at a deep portion of the particles 15A and 15B, the amount of the charge controlling material 153 to be added at the time of preparing the particles 15A and 15B is not proportional to the electrification capacity of the particles 15A and 15B. That is, the electrification amount of the charged particles 150 is irregular thereby to have a difficulty in controlling an amount of a voltage to be inputted to the electronic paper display device.
In order to solve the problem, a voltage to be applied to the display device has to be increased. However, when a higher voltage is applied to the display device, technical advantages of the electronic paper display device are lost.